This invention relates to a phased array radar and in particular to an apparatus and method for providing wideband interference suppression using subarray weighting to produce an array pattern having a notch in the direction of the interference.
Military radars must operate in a hostile environment, where they may be subjected to deliberate interference designed to degrade their performance. To suppress such ECM (Electronic-Counter-Measure) interference in a typical solid state phased array radar, a predetermined complex weight in terms of amplitude and phase is applied to each transmit/receive (T/R) module at the element level, so as to produce an array pattern having a notch in the direction of the interference. Such an interference suppressor uses a simple open-loop scheme which has no expensive real-time processor for interference suppression. However, it needs to know the direction of the interference which usually is given in most ECM threat scenarios.
In U.S. Pat. No. 4,872,016, entitled "Data Processing System For A Phased Array Antenna," issued Oct. 3, 1989, to Robert W. Kress, and assigned to Grumman Aerospace Corporation, it is pointed out that interference suppression is obtained in a phased array antenna system by generating nulls in the receive antenna pattern in the direction of the interference. The nulls are produced by adjusting the phase and amplitude (weight) of the received signal from each array element just enough to null the interference with minimal impact on the rest of the antenna pattern. This patent for interference suppression uses a close-loop microwave hardware and expensive data processors for real-time processing. As a result, its degrees of freedom is significantly limited by the number of array elements and the data processing load.
Conventional open-loop interference suppression algorithms generate complex weightings using full-aperture, element-level notching. A typical narrowband notched pattern using this method (designed to have -55 dB notch level) is shown in FIG. 5. For wideband operation, the typical solid state phased array is designed to use phase steer at the element level and time delay steer at the subarray level. For array scans off boresight, this results in correlated phase errors at frequencies offset from the center frequency of the wideband waveform. The phase slope within each subarray at frequencies either higher or lower than the center frequency is different from that at the center frequency. This is due to the phase steering at the element level. As a result of these correlated phase errors, high quantization lobes are produced in the notch section at the offset frequencies. (At center frequency the pattern is identical to the full aperture notched pattern of FIG. 5). By averaging the antenna patterns across the wideband waveform, a wideband notch level of -42 dB is obtained (FIG. 6) and is degraded by 13 dB as compared to the narrowband notch level of -55 dB (FIG. 5).
Using a beamformer architecture as shown in FIG. 4 (for monopulse operation), the sum beam is formed using Taylor weighting at the element level with a uniform combiner. Simultaneously, a Bayliss difference beam is formed using the same Taylor weighting at the element level together with a Bayliss/Taylor non-uniform combiner. The resultant monopulse patterns are shown in FIG. 7. If using narrowband notch weights per the conventional algorithm (full aperture notching), the sum beam exhibits a good notched pattern, but the difference beam is significantly degraded in terms of notch degradation, high angular errors, and decreased monopulse slope. This occurs even at mid-band and is illustrated in FIG. 8.
This invention is directed at providing full recovery of notch integrity for wideband interference suppression by using subarray weighting with minimal impact to the monopulse antenna patterns of the sum and difference beams for precision tracking.